Relatively little is known about the longitudinal course of neuropsychological abnormalities among patients with dementia syndromes. There is a lack of systematic longitudinal research examining the cognitive defects in dementia patients in relation to pre-existing and co-morbid psychiatric symptoms and syndromes, and neurological and medical diseases. This RSDA will allow the investigator to focus his research efforts in studying the natural history of dementia using a conceptually driven multifactorial approach. Using an existing database of 181 Alzheimer's disease patients, the occurrence, course and predictors of neuropsychological impairment will be determined. The role of psychiatric and neurological signs, symptoms, and syndromes in predicting the presence and pattern of cognitive deficits will be evaluated. Second, using data from an ongoing longitudinal study of the neuro-psychological abnormalities among HIV+ individuals seeking primary medical care, analyses will follow from a specific conceptual model of the neuropsychiatric predictors of presentation and investigate the functional CNS changes in dementia with Positron Emission Tomography. These studies will provide critical information about the natural history of dementia syndromes in the context of their overall neurobehavioral syndromes. The analyses will permit inferences about the nature of the neuro-anatomical defects associated with the observed syndromes, which will permit a more rational approach to evaluating therapeutics.